[PATCH 07/11] completion: use 'awk' to strip trailing path components

During git-aware path completion we complete one path component at a time, i.e. 'git add <TAB>' offers only 'dir/' at first, not 'dir/subdir/file' right away, just like Bash's own filename completion. However, since both 'git ls-files' and 'git diff-index' dive deep into subdirectories, we have to strip all trailing path components from the listed paths, keeping only the leading path component. This stripping is currently done in a shell loop in __git_index_files(), which can take a significant amount of time when it has to iterate through a large number of paths.

Replace this shell loop with a little 'awk' script using '/' as input field separator and printing the first field, which produces the same output much faster.

Listing all tracked files (12) and directories (23) at the top of the worktree in linux.git (over 62k files), i.e. what's doing all the hard work behind 'git rm <TAB>':

  Before this patch, best of five, using GNU awk on Linux:

    $ time cur= __git_complete_index_file

    real    0m2.149s
    user    0m1.307s
    sys     0m1.086s

  After:

    real    0m0.067s
    user    0m0.089s
    sys     0m0.023s

  Difference: -96.9%
  Speedup:     32.1x

Note that this could be done with 'sed', or even with 'cut', just as well, but the upcoming patches require 'awk's scriptability.

Note also that this change means one more fork()+exec()ed process during path completion, adding more overhead especially on Windows, but a later patch will more than make up for it by eliminating two other processes in the same function.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
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 contrib/completion/git-completion.bash | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)